The present invention relates to an image forming apparatus such as a laser beam printer, a copying machine, a facsimile machine or the like, with which an image is formed through an image formation process including a step of uniformly charging an image bearing member such as a dielectric member for electrostatic recording to a predetermined potential of a predetermined polarity.
More particularly, the present invention relates to an image forming apparatus which is of a contact charging and cleanerless type.
In the field of an image forming apparatus using an electrophotographic type, for example, a so-called cleanerless type image forming apparatus is known in which developing means also functions as cleaning means for removing residual toner still remaining on a surface of a photosensitive drum after toner image transfer onto a transfer material without using a particular cleaning means for such a purpose, by which the apparatus is downsized; the damage to the environmental health is suppressed because of the non-protection of waste toner; the lifetime of the photosensitive drum (image bearing member) is extended; the amount of the toner (developer) concerned for per page image formation is reduced.
(A) In such a cleanerless type image forming apparatus, an example of charging means is disclosed in Japanese Laid-open Patent Application 04-20986, for example. The charging means comprises a rotatable brush (elastic electroconductive member) as a contact charging member, which is contacted to the photosensitive drum to stir the untransferred toner remaining on the photosensitive drum to assist pattern destruction, and a DC voltage and an AC voltage are applied to uniformly charge the surface of photosensitive drum using this electric charge. Referring first to FIG. 12, there is schematically shown a peak-to-peak voltage of the AC voltage and a charging efficiency. In FIG. 12a, point L is a stable discharge point, and the electric discharge is stable in the range of the peak-to-peak voltage above point L, and the charged potential of the photosensitive drum is substantially the same as the DC voltage applied thereto (converged or saturated voltage). In view of the convergence or saturation of the charged potential in the range over the peak-to-peak voltage at the stabilization discharge point, the peak-to-peak voltage with which the charged potential exhibits the convergence is called xe2x80x9ccharged potential convergence voltagexe2x80x9d.
With the charging means of such a type in which a DC component biased with an AC component is applied to the elastic electroconductive member, the peak-to-peak voltage applied thereto is larger than the charged potential convergence voltage L for the purpose of stabilizing the charging action.
(B) On the other hand, Japanese Laid-open Patent Application 10-307455 discloses a simultaneous development and cleaning type image forming apparatus which is also of a cleanerless type, in which charging means is a direct charging type which does not use the electric discharge. More particularly, electroconductive charging-promotion particles are provided in the contact portion between the photosensitive drum and the contact charging member, and only a DC voltage is applied, by which a surface potential on the photosensitive drum is substantially the same as the DC voltage applied to the contact charging member. This system does not positively use the direct discharge phenomenon, and therefore, no ozone is produced. For the same reason, the deposition of electric discharge product onto the photosensitive drum can be suppressed, so that program of image flow under the high temperature and high humidity ambience can be avoided.
However, the following problem may arise in a cleanerless type image forming apparatus using (a) a rotatable brush (discharge function) or the charging means of the direct charging type.
With case (A), the toner is accumulated in the rotatable brush (contact charging member) with repetition of image forming operations with the result of improper charging.
The reason is as follows. The regular charging polarity of the toner (developer) is assumed as being negative, here. At that time when the untransferred toner passes through the transferring means, the toner particles are mixture of the toner particles charged to the positive polarity and the toner particles charged to the negative polarity. The amount of the toner particles charged to the positive polarity may be larger than the amount of the toner particles charged to the negative polarity.
The unstransferred toner contains so-called reversely charged toner particles, which are charged to the polarity (positive polarity toner) opposite to the regular polarity. These toner particles are attracted to the rotatable brush because of the electric field between the rotatable brush and the photosensitive drum.
The untransferred toner deposited on the rotatable brush enters the discharge region where the rotatable brush is opposed to the photosensitive drum, by the rotation of the rotatable brush.
In the discharge region, the electric discharge takes place with the results of production of positive charge and negative charge. The negative charge is attracted to the photosensitive drum to contribute to charging of the surface of the photosensitive drum. The simultaneously produced positive charge is attracted to the rotatable brush. At this time, the untransferred toner particles are deposited on the surface of the rotatable brush, and the untransferred toner particles are further electrically charged to the positive polarity.
Even if the untransferred toner charge to the positive polarity passes through the contact portion between the rotatable brush and the photosensitive drum and through the discharge region downstream of the contact portion with respect to the rotational direction, they are attracted toward the rotatable brush side due to the electric field formed between the rotatable brush and the photosensitive drum, and therefore, they are kept deposited on the charge member.
By repeating the foregoing operation, the toner deposited on the rotatable brush is charging strongly to the positive polarity. In addition, additional untransferred toner is accumulated even to such an extent that the charging member becomes unable to charge the photosensitive drum to the regular surface potential, with the result of improper charging.
When the AC voltage is sufficiently large, the discharge stably takes place, and the amount of electric discharge is large, and therefore, electric discharge product is deposited on the photosensitive drum. The electric discharge product thus deposited on the photosensitive drum exhibits low resistance under a high temperature and high humidity ambience, and therefore, image defect such as image flow tends to occur.
This is not limited to the rotatable brush, the above-described image defect results even in the case of the charging means using a contact charging member which relies on discharge function, for the same reasons.
When use is made of the charging means of the direct charging type as in case (B), the amount of the untransferred toner deposited on the charging means increases with the increase of the number of processings with a result of contamination of the charging means, and therefore improper charging and the image defect arise.
The reason is that in the case of the direct charging type charging means, the DC voltage applied to the charging means and the charged potential of the surface of the photosensitive drum are substantially the same, and therefore, the untransferred toner deposited on the charging means is not electrostatically delivered out of the charging means. The image defect is remarkable when a peripheral speed difference is provided between the photosensitive drum and the charging means so as to positively remove the untransferred toner from the photosensitive drum.
Japanese Laid-open Patent Application 11-149205 discloses that AC bias is superimposed to the DC bias during the non-image-formation period so as to deliver the toner from the charging means to the surface of the photosensitive drum.
However, when the AC bias (having a peak-to-peak voltage 200V, a frequency and a rectangular wave shape) is incorporated in an image forming apparatus using a non-magnetic toner, the untransferred toner delivering power is not enough with the result that so-called drum ghost image (image deterioration as a result of influence of an image hysteresis) with the increase of the number of prints. It is difficult to clean the charging means.
It to be possible to raise the peak-to-peak voltage simultaneously with the application of the AC bias also during the image formation, in order to prevent the drum ghost image. However, if this is done, the following image defect results.
When a halftone image is formed, a discrete single dot is not correctly reproduced as a single dot, but is reproduced as scattered dots. Such an image defect occurs remarkably in a high light range, that is, reflection density is 0.5 or less as measured by (Macbeth 1200) reflection density meter.
Accordingly, it is a principal object of the present invention to provide an image forming apparatus in which contamination of a charge member is effectively prevented. It is another object of the present invention to provide an image forming apparatus in which non-uniformity in charging is avoided, and therefore, image defect can be avoided. It is a further object of the present invention to provide an image forming apparatus and which a production of a ghost image can be avoided.